Èñòî÷íèê: wikipedia
A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor.[1] A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults.[2]
Every electric motor has to have some sort of controller. The motor controller will have differing features and complexity depending on the task that the motor will be performing. The simplest case is a switch to connect a motor to a power source, such as in small appliances or power tools. The switch may be manually operated or may be a relay or contactor connected to some form of sensor to automatically start and stop the motor. The switch may have several positions to select different connections of the motor. This may allow reduced-voltage starting of the motor, reversing control or selection of multiple speeds. Overload and overcurrent protection may be omitted in very small motor controllers, which rely on the supplying circuit to have overcurrent protection. Small motors may have built-in overload devices to automatically open the circuit on overload. Larger motors have a protective overload relay or temperature sensing relay included in the controller and fuses or circuit breakers for overcurrent protection. An automatic motor controller may also include limit switches or other devices to protect the driven machinery.
More complex motor controllers may be used to accurately control the speed and torque of the connected motor (or motors) and may be part of closed loop control systems for precise positioning of a driven machine. For example, a numerically-controlled lathe will accurately position the cutting tool according to a preprogrammed profile and compensate for varying load conditions and perturbing forces to maintain tool position.
Motor controllers can be manually, remotely or automatically operated. They may include only the means for starting and stopping the motor or they may include other functions.[2][3][4]
An electric motor controller can be classified by the type of motor it is to drive such as permanent magnet, servo, series, separately excited, and alternating current. A motor controller is connected to a power source such as a battery pack or power supply, and control circuitry in the form of analog or digital input signals.
A small motor can be started by simply plugging it into an electrical receptacle or by using a switch or circuit breaker. A larger motor requires a specialized switching unit called a motor starter or motor contactor. When energized, a direct on line (DOL) starter immediately connects the motor terminals directly to the power supply. A motor soft starter connects the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.[2][3][4]
An adjustable-speed drive (ASD) or variable-speed drive (VSD) is an interconnected combination of equipment that provides a means of driving and adjusting the operating speed of a mechanical load. An electrical adjustable-speed drive consists of an electric motor and a speed controller or power converter plus auxiliary devices and equipment. In common usage, the term “drive” is often applied to just the controller.[3][4]
Recent developments in drive electronics have allowed efficient and convenient speed control of these motors, where this has not traditionally been the case. The newest advancements allow for torque generation down to zero speed. This allows the polyphase AC induction motor to compete in areas where DC motors have long dominated, and presents an advantage in robustness of design, cost, and reduced maintenance.[4]
Phase vector drives (or simply vector drives) are an improvement over variable frequency drives (VFDs) in that they separate the calculations of magnetizing current and torque generating current. These quantities are represented by phase vectors, and are combined to produce the driving phase vector which in turn is decomposed into the driving components of the output stage. These calculations need a fast microprocessor, typically a DSP device.
Unlike a VFD, a vector drive is a closed loop system. It takes feedback on rotor position and phase currents. Rotor position can be obtained through an encoder, but is often sensed by the reverse EMF generated on the motor leads.
In some configurations, a vector drive may be able to generate full rated motor torque at zero speed.
Direct torque control has better torque control dynamics than the PI-current controller based vector control. Thus it suits better to servo control applications. However, it has some advantage over other control methods in other applications as well because due to the faster control it has better capabilities to damp mechanical resonances and thus extend the life of the mechanical system.
SCR controls for DC motors convert AC power to direct current, with adjustable voltage. Small DC drives are common in industry, running from line voltages, with motors rated at 90V for 120V line, and 180V for a 240V line. Larger drives, up to thousands of horsepower, are powered by three phase supplies and are used in such applications as rolling mills, paper machines, excavators, and ship propulsion. DC drivers are available in reversing and non-reversing models. The waveform of the current through the motor by a single-phase drive will have strong ripple components due to the switching at line frequency. This can be reduced by use of a polyphase supply or smoothing inductors in the motor circuit; otherwise the ripple currents produce motor heating, excess noise, and loss of motor torque.
PWM controls use pulse width modulation to regulate the current sent to the motor. Unlike SCR controls which switch at line frequency, PWM controls produce smoother current at higher switching frequencies, typically between 1 and 20 kHz. At 20 kHz, the switching frequency is inaudible to humans, thereby eliminating the hum which switching at lower frequency produces. However, some motor controllers for radio controlled models make use of the motor to produce audible sound, most commonly simple beeps.
A PWM controller typically contains a large reservoir capacitor and an H-bridge arrangement of switching elements (thyristors, Mosfets, solid state relays, or transistors).
A stepper, or stepping, motor is a synchronous, brushless, high pole count, polyphase motor. Control is usually, but not exclusively, done open loop, i.e. the rotor position is assumed to follow a controlled rotating field. Because of this, precise positioning with steppers is simpler and cheaper than closed loop controls.
Modern stepper controllers drive the motor with much higher voltages than the motor nameplate rated voltage, and limit current through chopping. The usual setup is to have a positioning controller, known as an indexer, sending step and direction pulses to a separate higher voltage drive circuit which is responsible for commutation and current limiting.
DC motors are typically controlled by using a transistor configuration called an "H-bridge". This consists of a minimum of four mechanical or solid-state switches, such as two NPN and two PNP transistors. One NPN and one PNP transistor are activated at a time. Both NPN or PNP transistors can be activated to cause a short across the motor terminals, which can be useful for slowing down the motor from the back EMF it creates.